The Brain’s Secret to Sleeping Like a Log

In this clamorous modern world, heavy sleepers have an advantage: They can snooze despite noisy neighbors and car alarms, and they’re capable of conking out on a red-eye flight to awake refreshed and smiling.

But how do these sound sleepers do it? According to a neuroscience study published today in Current Biology, they’re blessed with a type of brain activity that may essentially block out noise.

Sleep researchers from Harvard Medical School performed a slightly torturous experiment on 12 healthy volunteers. On their first night at the sleep lab, the subjects’ brain waves were monitored via electroencephalography (EEG), but they were otherwise left in peace. That night, the researchers measured one particular sleep phenomenon: the brief bursts of high-frequency waves known as “sleep spindles.” On the following two nights, the researchers did their best to replicate a really irritating night’s sleep.

“The volunteers would come in and we’d show them this luxury environment with a queen bed and comfy sheets, but there are these four very large speakers pointed straight at their heads,” said study coauthor Jeffrey Ellenbogen.

The sleepers were then exposed to a steady stream of auditory assaults. Each sound — whether it was a phone ringing, an animated conversation, or a jet engine — would be played quietly at first, and then gradually cranked up until the patients’ brain waves showed a disruption to the sleep pattern. After a few seconds of quiet, the researchers cued up the next sound and the process began again.

The results showed that patients with more frequent sleep spindles were better able to tolerate noise; it took higher-decibel sounds to disrupt their sleep patterns. Ellenbogen says this gives researchers a new predictive power.

“If you know how many spindles a person is producing and compare them to others, you can predict who among them will run into trouble when it comes to blocking sound during sleep,” he said.

Ellenbogen and his colleagues believe the sleep spindles’ protective effect relates to where they’re produced: the thalamus, the brain region that acts as a waystation for sounds and other sensory information on their way to the perceptual areas of the brain. The sleep spindles may be “colliding” with the sounds and blocking their progress.

“The mechanism that produces spindles may actually interfere with the transmission of sensory information through the thalamus to the cortex,” said study coauthor Thien Thanh Dang-Vu.

In people who produce more frequent spindles, there’s a better chance that a noise will encounter this sensory blockade.

It isn’t yet known why some people naturally produce more sleep spindles than others, nor is it clear if spindles can be artificially boosted to help people such as hospital patients sleep in noisy environments. The researchers say sleeping pills do increase spindles, but they may interfere with other beneficial sleep stages.

The new work also has implications for learning and memory research. Stuart Fogel, who studies sleep at the University of Montreal and who was not involved in the current study, says the research complements previous studies that have found a link between a high frequency of sleep spindles and high performance on certain IQ and memory tests.

“It raises one of the unanswered questions from our research,” Fogel says. “Do we see these correlations between IQ and memory consolidation because sleep spindles serve a protective function, and those with better quality sleep can learn more easily? Or do spindles also play a more active role in memory consolidation?”

Ellenbogen says it’s certainly conceivable that spindles play a dual role. Regardless, he says, his study does add to the evidence of how busy the sleeping brain is.

“Although our computer vernacular uses ‘sleep’ to refer to a process of temporary shut-down, that’s not the way our brain works,” Ellenbogen wrote in an email to Wired.com. “During sleep, our neurons are busy doing very complicated processing, including, this study shows, generating sleep spindles to protect us from being awoken from noises in the environment.”